1. Field of the Industrial Application
The present invention relates to a servo motor control device.
2. Description of the Prior Art
FIG. 3 shows one example of a conventional servo motor control device.
In the example, in order to rotate a motor 1 at a speed .omega. [rad/sec], a target value .omega. for the speed .omega.* is determined, and a speed instruction signal V.sub.1 [V] is applied in compliance with the target value .omega.*. Accordingly, the speed instruction signal V.sub.1 is correlated with the speed .omega. of the motor 1. The speed instruction signal has a trapezoid waveform as shown in FIG. 4. In the case where the motor 1 is used to drive an arm of an industrial robOt, the above-described trapezoid waveform speed instruction signal V.sub.1 is repeatedly inputted in order to reciprocate the arm of the robot. In the operation of the motor 1, the speed .omega. [rad/sec] thereof is applied to a speed detector 2, where it is subjected to F-V (frequency-to-velocity) conversion. That is, it is converted into .omega.K.sub.TG, which is fed back to a subtractor 3 where it is subtracted from the speed instruction signal V.sub.1 (where K.sub.TG has the dimension [V/rad/sec], and .omega.K.sub.TG has the dimension [V]).
The output (V.sub.1 -.omega.K.sub.TG) of the subtractor 3 is applied to a PI (proportional integral) compensator 4 having the following transfer characteristic: EQU Kv(1+1/(S Tv))
The transfer function G(s) of the PI compensator 4 is as follows: ##EQU1## where S=j .omega., and therefore EQU G(j.omega.)=Kv(1+j.omega.Tv)/(j.omega.T.sub.v)
The gain characteristic (g) is: ##EQU2##
The function of equation (1) is as shown in FIG. 5. That is, the PI compensator 4 offers large gain with respect to the low frequency component of the difference output of the subtractor 3, and offers constant gain with respect to the high frequency component higher than Tv of the output of the subtractor 3.
The output of the PI compensator 4 is a current instruction signal (the dimension of V) which is applied as a positive input to the point A. In addition, the output of the PI compensator 4 is provided as a torque indication voltage. The current I [A] flowing in the motor 1 is applied to a current detector 5, where it is converted into a voltage which is applied as a negative input I K.sub.If to the point A. The input K.sub.If is measured in [V/A], and the input I K.sub.If is measured in [V]. At the point A, the output of the current detector 5 is subtracted from the output of the PI compensator 4, and the resultant difference is applied to a current amplifier 6, which is multiplied by a factor K.sub.I. K.sub.I has no dimension. The current amplifier 6 supplies to the motor 1 a drive voltage (the output of a drive transistor) for actually driving the motor 1. Upon application of the output of the current amplifier 6 to the motor 1, current flows in the resistance R of the motor 1. A voltage (j .omega. L.times.I) attributed to the inductance L of the motor 1 is applied, as a negative input, to the point B, where it is subtracted from the output of the current amplifier 6. The voltage drop V of the motor is as follows: V=R I+S L I. Therefore, R I=V-S L I. A larger part of the high frequency component of the voltage V is absorbed by the inductance L, and R I, as shown in FIG. 6, and does not contain many high frequency components. Also, S L I=j .omega. L I. Therefore, when the frequency f of .omega. increases, j.omega.L I is increased, whereas R I is decreased.
The dimension of the torque constant K.sub.T of the motor 1 is [Kg.multidot.m.sup.2 .multidot.sec.sup.-2 /A], I K.sub.T has the dimension [Kg.multidot.m.sup.2 .multidot.sec.sup.-2 ], and torque T has the dimension of [Kg.multidot.m.sup.2 .multidot.sec.sup.-2 ]. The transfer function due to inertia J of the motor 1 is 1/(S J), and T.times.(1/SJ)=.omega. [rad/sec] is the speed of rotation .omega. of the motor 1, where S represents d/dt which has the dimension of [sec.sup.-1 ], and J is represented by the dimension of [Kg.multidot.m.sup.2 ]. Therefore, the following is established: ##EQU3##
That is, 1/V .intg.T(t) dt=.omega.(t) Since [rad] has no dimension, T.multidot.(1/(S J)) may be represented by [rad/sec].
When the motor 1 rotates at a speed of .omega., a counter electromotive force is induced therein, and therefore the counter electromotive voltage .omega. K.sub.E [V] which is the product of the counter electromotive force constant K.sub.E [V/rad/sec] and the speed of .omega. is applied, as an input, to the point B, where it is subtracted from the output of the current amplifier 6. Hence, as the speed of rotation .omega. of the motor 1 increases, the counter electromotive force is increased. Further, the current flowing in the motor is inversely proportional to the counter electromotive force.
The output of the PI compensator 4 is provided, as the torque indication signal, at an output terminal (torque indication terminal). The torque indication signal can be observed with an oscilloscope or the like. The torque of the motor 1 can be read through observation of the torque indication signal. Therefore, when a device (such as an arm of an industrial robot) moves abnormally which is driven by the motor 1, it can be directly detected whether or not the motor provides torque as required. This provides the result that it can be detected whether or not the motor 1 or the device operates correctly. Furthermore, since the torque of the motor in operation can be visually detected, the operating characteristics of the motor can be detected. In addition, the torque indication signal may be utilized for control of the operation of the motor as follows. When the torque of the motor is abnormal, or higher and lower than the predetermined value, a motor stop signal may be provided by applying the torque indication signal to a robot controller or the like.
In the above-described servo motor control device, the counter electromotive force .omega.K.sub.E is applied at point B. This is equivalent to the application of (1/K.sub.I).multidot..omega.K.sub.E at point A. That is, FIG. 3 can be rewritten into FIG. 7. The actual torque of the motor 1 is proportional to the true instruction value I" which is obtained by subtracting a value Ie', proportional to the counter electromotive force, from the output I' of the PI compensator 4 as shown in FIG. 8. However, the torque indication voltage I' at the torque indication terminal is the sum of I" and (1/K.sub.I).multidot..omega.K.sub.E. The value (1/K.sub.I).multidot..omega.K.sub.E increases with the speed of rotation .omega., and accordingly the torque indication voltage I' increases with the speed of rotation .omega., as shown in FIG. 9. Thus, the torque indication voltage I' corresponds to the torque of the motor 1 in the rate of 1:1. However, when the torque indication voltage I' is changed by the speed of rotation .omega., the former I' is shifted as shown in FIG. 9.